The invention relates to a unipolar multiple-wavelength quantum-well laser.
In a standard semiconductor laser, the emission of photons occurs during a recombination between an electron of the conduction band and a hole of the valence band. FIG. 1 gives a schematic exemplary view of the case of a quantum well laser. In this case, the transmission wavelength is simply fixed by the energy of the transition between the two fundamental levels E1 and HH1. This energy may be modified for example by changing the width of the quantum well. However, if several quantum wells emitting at different wavelengths are placed in the cavity of a semiconductor laser, the total system will not emit laser radiation at all these different wavelengths simultaneously. Indeed there will occur an effect of competition between the different wells. Only one will prevail and the device will have emission solely at the wavelength of the "victor". This can be explained as follows: if the injection current is increased, once the laser threshold is crossed for only one of these wells, the device is in laser operation mode with stimulated emission which very soon empties these wells and therefore pumps the totality of the carriers (electrons and holes) that are injected and then lost during the radiative recombination. The Fermi level is therefore blocked and no longer increases when the current is increased. The other wells therefore cannot increase their carrier density and will not reach their laser operation threshold. These effects are well known in the literature (see appended document [1] at the end of the description).
Quantum cascade lasers (QCL) appeared in 1994 (see documents [2] and [3] at the end of the description) have a major difference with these standard lasers: the emission of a photon is done within only one band and implies only one type of carrier. This is why these QCLs may also be called "unipolar lasers". In operating in the conduction band, this type of carrier is the electron but it is possible to envisage transitions within the valence band. An operation of this kind is called intra-band operation. During operation in the conduction band, the emission involves only electrons. There is no disappearance of carriers during the emission of a photon as is the case with standard lasers. The same electron may furthermore be used several times in several quantum wells in crossing the structure, and may thus emit several photons.
The invention takes advantage of this property so that one and the same electron is used several times in several quantum wells.